With the increasing requirements on the capacity and flexibility of optical communication systems, the coherent optical communication technology has become increasingly important. In comparison with incoherent technology such as on-off key (OOK) or self-coherent technology such as differential quadrature phase shift key (DQPSK), the coherent technology has the following advantages: it has 3 dB optical signal to noise ratio (OSNR) gain; it is convenient to utilize equalization technology; and it is possible to employ more efficient modulation technologies such as quadrature amplitude modulation (QAM).
Like electrical coherent technology, the optical coherent receiver also needs a device to recover the carrier phase. This can be realized by using an analog phase locked loop, as explained by Leonid G. Kazovsky in “Decision-Driven Phase-Locked Loop for Optical Homodyne Receivers”, IEEE/OSA Journal of Lightwave Technology, Vol. LT-3, No. 6, December, 1985, P 1238-1247. As shown in FIG. 1, the analog phase locked loop comprises a phase estimator 101, a loop filter 102 and a voltage-controlled oscillator (VCO) 103, also referred to as local oscillator laser. The analog phase locked loop is relatively low in speed due to its inherent loop delay. Moreover, such an analog phase locked loop has the following disadvantages: low control speed due to its long loop delay; high requirements on the phase noise of the carrier and the voltage-controlled oscillator; and large phase error caused by the phase noise, etc.
With the rapid development of the technology of electronic devices in recent years, digital technology has been increasingly employed in optical communications. Dany-Sebastien Ly-Gagnon et al. demonstrated an optical coherent receiver making use of the digital signal processing technology in OFC2005 OTuL4. They used feed-forward phase estimation instead of feedback phase locked loop. FIG. 2 shows such a method. As shown in FIG. 2, the optical coherent receiver comprises a local oscillator laser for supplying a local oscillator optical signal, an optical 90 degree frequency mixer for mixing a received optical signal with the local oscillator optical signal, first and second balancing photoelectric detectors for converting the optical signals outputted from the optical 90 degree frequency mixer into baseband electrical signals; an analog to digital converter (ADC) 201, an argument calculator 202, a decoder 203, and a phase estimator 204.
A first input terminal of the optical 90 degree frequency mixer is connected to an optical input, a second input terminal thereof is connected to an output of the local oscillator laser, and first and second output terminals thereof are respectively connected to input terminals of the first and the second balancing photoelectric detectors; output terminals of the first and the second balancing photoelectric detectors are respectively connected to first and second input terminals of the analog to digital converter 201; first and second output terminals of the analog to digital converter 201 are respectively connected to first and second input terminals of the phase estimator 204 and first and second input terminals of the argument calculator 202; an output terminal of the argument calculator 202 is connected to a first input terminal of the decoder 203, and an output terminal of the phase estimator 204 is connected to a second input terminal of the decoder 203.
The analog to digital converter 201 converts analog cophase signal (I) and quadrature signal (Q) into a digital signal I+jQ, which is a complex signal. The argument calculator 202 obtains the argument, namely the phase, of the complex signal. The phase estimator 204 obtains a phase difference between a carrier signal of the received optical signal and the local oscillator optical signal. The decoder 203 subtracts the phase difference estimated by the phase estimator 204 from the output of the argument calculator 202 to recover the transmitted data.
As shown in FIG. 2, the phase estimator 204 comprises a four times power calculator 205, an averager 207, an argument calculator 206, and a dividing by four calculator 208. First and second input terminals of the four times power calculator 205 are respectively connected to first and second output terminals of the analog to digital converter 201, an output terminal of the four times power calculator 205 is connected to an input terminal of the averager 207, an output terminal of the averager 207 is connected to an input terminal of the argument calculator 206, an output terminal of the argument calculator 206 is connected to an input terminal of the dividing by four calculator 208, and an output terminal of the dividing by four calculator 208 is connected to a second input terminal of the decoder 203.
As can be seen, all of the above calculations are carried out in the digital domain. Here, the digital feed-forward phase estimator is used to replace the analog phase locked loop in the previous optical coherent systems to avoid the defects of the analog phase locked loop as discussed above. However, this method is applicable merely for the phase shift keying (PSK) modulation mode, because the basic principle of this method rests in the subtraction of two phases. The method cannot be applied in more advanced modulation technologies (such as the QAM), whereas such a defect does not exist in the solution of the phase locked loop. On the other hand, all of the four times power calculator 205 and the argument calculators 202 and 206 perform nonlinear computations, and it is very complicated to realize these nonlinear computations by means of hardware or digital signal processing technology.
In view of the aforementioned circumstances, there is currently a pressing need for a novel phase control technique that combines the advantages of the phase locked loop and the digital signal processing technology.